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Cambridge IGCSE Biology · 0610

Chapter 13: Excretion in humans

Definitions and waste products

Excretion
The removal of waste products of metabolism and substances in excess of requirements from organisms.
Carbon dioxide
Produced in cells during respiration; it exits cells, dissolves in the blood, and is carried to the lungs to be excreted.
Urea
A nitrogenous waste product formed in the liver from an excess of amino acids.
Excess water and ions
Filtered from the blood by the kidneys and excreted as urine.

Exam Traps

  • Do not confuse excretion with egestion — faeces removal is egestion, not excretion.
  • Avoid saying urea is formed in the kidneys — it is made in the liver.

The role of the liver

Assimilation
The liver converts absorbed amino acids into proteins for use in the body.
Urea formation
Amino acids cannot be stored in the body (unlike glucose); therefore, excess amino acids that cannot be converted to proteins are processed in the liver to form urea.
Deamination
The process in the liver involving the removal of the nitrogen-containing part of amino acids to form urea.
Toxicity
Excretion is vital because a build-up of urea is toxic and could cause harm to the organism.

Exam Traps

  • Do not say excess amino acids are stored like glycogen — they are deaminated to urea.
  • Avoid describing deamination as filtration at the glomerulus — that occurs in the kidney nephron.

The human renal system

You must be able to identify these structures in diagrams and images:

  1. Kidneys: Two organs that filter waste and excess substances from the blood to be excreted as urine.
  2. Ureters: Tubes that transport urine from the kidneys to the bladder.
  3. Bladder: A muscular sac where urine is stored before being excreted.
  4. Urethra: The tube through which urine passes from the bladder out of the body.
Blood supply
Blood enters the kidney through the renal artery (containing waste products) and exits via the renal vein (containing clean blood).
The human renal system showing kidneys, renal artery, renal vein, ureters, bladder, and urethra
Diagram 1: The human renal system. Clear labels of the kidneys, renal artery, renal vein, ureters, bladder, and urethra.

Exam Traps

  • Do not reverse renal artery and renal vein — artery brings waste in, vein takes clean blood out.
  • Avoid saying the bladder filters blood — kidneys filter; the bladder only stores urine.

Kidney structure and the nephron

Internal structure
The kidney has two main identifiable regions:
  • Cortex: The outer region of the kidney, which contains the nephrons.
  • Medulla: The inner region of the kidney.
Nephron
The functional unit of the kidney, with millions present in each organ.
Filtration at the glomerulus
The glomerulus is a cluster of capillaries at the beginning of the nephron where water, glucose, urea, and ions are filtered out of the blood.
Selective reabsorption
As the filtered fluid moves along the nephron tubule, the body reabsorbs useful substances back into the blood capillaries:
  • All of the glucose is reabsorbed.
  • Some of the ions are reabsorbed.
  • Most of the water is reabsorbed.
Urine formation
The remaining fluid in the nephron, consisting of urea, excess water, and excess ions, forms urine.
Cross-section of a kidney showing cortex and medulla with nephron inset showing glomerulus, tubule, filtration, and selective reabsorption
Diagram 2: Structure of a kidney and nephron. A cross-section of the kidney shows the cortex and medulla. An inset shows a nephron with labels for the glomerulus and tubule, using arrows to illustrate filtration into the nephron and selective reabsorption back into the blood.

Exam Traps

  • Do not say all filtrate is reabsorbed — urea and excess water/ions stay in urine.
  • Avoid claiming glucose appears in urine of healthy people — all glucose is reabsorbed.

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